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Magnetic Properties of Rocks from the South-Eastern Part Ofthe Weddell Sea Region, Antarctica

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Magnetic Properties of Rocks from the South-Eastern Part Ofthe Weddell Sea Region, Antarctica Polarforschung 67 (3): 119 -124,1997 (erschienen 2000) Magnetic Properties of Rocks from the South-Eastern Part ofthe Weddell Sea Region, Antarctica By Michael B. Sergeyev' Summary: The major objective of this paper is to sumrnarize the available The aim of this paper is to summarize all available petro­ data on magnetic properties of rocks outcropping in the south-eastern part of magnetic data from the south-eastern part of the Weddell Sea the Weddell Sea region. From north to south: western Dronning Maud Land, the coastal nunataks of north-western Coats Land, the Theron Mountains, the region, from western Dronning Maud Land in the north-east to Shackleton Range, the Whichaway Nunataks and the Pensacola Mountains. the Pensacola Mountains in the south-west (Fig. 1) and to Although the quality of data from the different areas varies, such a summary identify which lithological units may cause the magnetic allows to identify the main lithological units responsible for the magnetic anomalies. Predominantly, they represent sequences of Precambrian felsic anomalies in this area. gneisses and Jurassie intrusives of different composition. Zusammenfassung: Das Hauptziel dieses Artikels ist die Zusammenfassung WESTERN DRONNING MAUD LAND der verfügbaren Daten der magnetischen Eigenschaften von Gesteinen, die im südöstlichen Abschnitt der Weddellsee-Region aufgeschlossen sind. Von Norden nach Süden: westliches Dronning Maud Land, die küstennahen The preliminary study of magnetic properties of the rocks of Nunataks vom nordwestlichen Coats Land, die Theron Mountains, die Shack­ western Dronning Maud Land (Fig. 2) was carried out on leton Range, die Whichaway Nunataks und die Pensacola Mountains. Obwohl sampIes collected by Russian geologists during the 1987/88 die Datenqualität gebietsweise sehr unterschiedlich ist, erlaubt solch eine Zusammenfassung die Identifikation der wichtigsten lithologischen austral summer season. Most of the available sampIes were Einheiten, die ursächlich sind für die magnetischen Anomalien. Sie repräsen• collected in Borgmassivet and Ahlmannryggen, whereas other tieren hauptsächlich Sequenzen von präkambrischen felsischen Gneisen und areas were sampled only sporadically. jurassischen Intrusiva verschiedener Zusammensetzung. Western Dronning Maud Land consists of two tectonic provinces, the Grunehogna and Maudheim provinces INTRODUCTION (WOLMARANS & KENT 1982), that are very different in geology and lithology. The Grunehogna province situated to the north­ Though the region of the Weddell Sea and its mountainous west of the Jutulstraumen and Pencksökket is built up of a frame is difficult to access it is in some aspects a corn­ subhorizontal cover of Early-Mid Proterozoic low-grade paratively weil investigated part of Antarctica. It was covered metasediments and metavo1canics (Ahlmannryggen and by several aeromagnetic surveys carried out mainly by Jutulstraumen Groups) intruded by sills of Mid Proterozoic Russian and British expeditions (MASOLOV 1980, JOHNSON et dolerites (Borgmassivet intrusives). The highly-magnetic al. 1992, GOLYNSKY et al. 2000) and the geology of the nature is characteristic only for Mid Proterozoic dolerites mountainous areas free from ice was studied by numerous (Tab. 1), whereas all other units are generally low-rnagnetic, field parties (e.g. WOLMARANS & KENT 1982, CLARKSON et al. Only the magnetization of the andesitic basalts of the 1995). However, systematic studies of magnetic properties of Straumsnutane Formation (main member of Jutulstraumen rocks outcropping in this region were almost not conducted. Group) remains an open question: ten sampIes from this unit Measurements of rock magnetization either were sporadic 01' collected by Russian geologists were low-magnetic (Tab. 1), were carried out in the context of paleomagnetic studies. whereas three sampIes collected by South African geologists Exceptions were the investigations of ORLENKO (1982) and for paleomagnetic studies (WOLMARANS & KENT 1982) were MASLANYJ et al. (1991). The former was the first compre­ highly-magnetic (mean magnetic susceptibility X = 9.97 10.3 hensive petromagnetic study of rock sampIes from Antarctica in SI units). In any case, the total volume of highly-magnetic including only a few areas of the south-western part of the rocks within the outcrops of Grunehogna province can not be Weddell Sea region, the Pensacola Mountains and the large, because the anomaly pattern is "quiet" and the Shackleton Range. The latter was a study devoted mainly to amplitudes are small: from -100 to +250 nT (GOLYNSKY et al. the western part of the Weddell Sea region including some 2000). data from the Dufek intrusion located in the southernmost part of the region. The only systematic petromagnetic field study The Maudheim province, south-east of the Jutulstraumen and were carried out more recently in the Shackleton Range Pencksökket, is an area built up by polycyclically deformed (SERGEYEV et al. 1999). Mid Proterozoic high-grade metamorphics (Sverdrupfjella Group) represented predominantly by granitized and migmatized gneisses. These sequences are intruded by Phanerozoic felsic plutons of different composition and , VNIIOkeangcologia, Angliysky ave. 1,190121, Saint-Petcrsburg, Russia, Jurassie mafic magmatic rocks that are covered by Palaeozoic <[email protected]> sedimentary rocks (Early (?) Palaeozoic Urfjell Group and Manuscript received 10 July 1999, accepted 28 February 2000 Late Palaeozoic Amelang Formation) and Jurassie basalts. 119 During this study the highly-magnetic nature was revealed Vestfjella is predominantly made up of Jurassie basalts, base­ within the Maudheim province only for Mesozoic nepheline ment rocks are unknown, and it is unclear whether this area is syenites of Gburekspitzen and Jurassie basalts of part of Grunehogna province 01' part of Maudheim province. Kirwanveggen (Tab. 1). Both units are not very widespreael, Notably, Jurassie basalts from Vestfjella are Iow-rnagnetic anel, it is assumed that neither can be responsible for the (Tab. 1). Obvious1y, the number of basalt samples from here so-called Kirwan anomalies, which are the brightest and most (N = 4) do not give a reliab1e value. However, KRISTOFFERSEN important features of the magnetic field in this area. & AALERUD (1988) with reference to personal communication Phanerozoic felsic plutons of different composition (one of of R. Lovlie also noted the low magnetization of basalts in them is the pluton ofnepheline syenites of Gburekspitzen) are Vestfjella. Hence, the characteristic magnetic anomaly known not very extensive and, hence, can be responsible only for in the southern part of the Vestfjella area can not be caused by some comparative1y small anomalies in this area. The Jurassie basalts. Possibly, it is caused by a body of Jurassie olivine basalt bodies in western Dronning Maud Land are also not gabbro known from the southern part of Vestfjella. large enough to cause the Kirwan anomalies. Even in areas Unfortunate1y, this massif was not visited by Russian where these rocks crop out, not any high-amplitude anomaly geologists, and, hence, there are no petromagnetic data occurs. HUNTER et al. (1991) and CORNER et al. (1991) propose that a + causative body of the Kirwan anomalies correlates with the 70° granite-gneisses. LEITCHENKOV & KHLYUPIN (1993) also suggest that the Kirwan anomalies correlate with the old WEDDELL Precambrian complexes and emphasize that the magnetic SEA causative body most Iikely is not connected with mafic rocks, because it has no gravity expression. Highly-magnetic felsic gneiss formations are quite usual members of Precambrian high-grade terrains. In particular, they are found in other parts of the Weddell Sea region: in the Haag Nunatak (MASLANYJ 72' et al. 1991) and the Shackleton Range (see below). Unfortunately, the rocks of the Sverdrupfjella Group are almost not represented in the available Russian sampIe collections, therefore respective petromagnetic data are absent. 200 km 7'10 S WEDDELLSEA .," ." MAUD + -+ 15° W 5° HH-/+ .,,J-' .. " ••-" tHH-I~ .j. -H.l - Archacun grunitc; COATS LAND LAND ~ - Eurly-Mid Protcrozoic lew-grade mctascdimcntary aud volcunic rocks (Ahlmnnnryggcn und Jutulstraumcn Groups) aud fVIid Protcrozoic Borgmassiver intrusivcs; ~ - Mid Protcrozoic high-grade mcuunorphics (Svcrdrupljclla Group ami analogucs): Theron •••• m:lTEl:l:l:1 Mountains ~ - Early ('1) Palucozoic lcrrigenous scdimcntury rocks (Urljcll Group); T ."-+ 0° I~:;/'::"';"".Shackleton m:nm - Laie l'ulacozoic tcrrigcnous scdimcntury rocks " Range (Amelung Plateau Formation and analogucs); .. ~ ... - Jurassie basalt: DM Whichaway I. Nunataks • Jurassie olivinc-gubbro; YtFR 90° '~~ Pensacola E Phaucrozoic Ielsic intrusions; ,, Mountains BI - - outcrops arcas. ..',' + -+ '/ Gb - Gburckspitzcn. 85° 'ti', 180° S Fig. 2: Geological sketch map of western Dronning Maud Land (modified 60° W 40° 20° 0° 20° E after WOLMARANS & KENT 1982, HJELLE & WINSNES 1972, TINGEY 1991). General simplified pattern ofmagnetic anomalies is shown in accordance with data of KHLYUPIN et al. (1987) und SPIRIDONOV et al. (1987). Contour interval Fig. 1: Location of outcrops in the south-eastern part of the Weddell Sea at 200 nT. region. L = Littlewood Nunatak; B = Bertrab Nunatak; DM = Dufek Massif; FR = Forrestal Range. Abb. 2: Skizzierte geologische Karte des westlichen Dronning Maud Land (verändert nach WOLMARANS & KENT 1982, HJELLE & WINSNES 1972, TINGEY Abb. 1: Lokation der Aufschlüsse im südöstlichen Abschnitt der Weddell­ 1991). Das vereinfachte
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